Lecture"LIPID METABOLISM"

TRANSFORM Research Institute LIPIDS IN THE PROCESS OF DIGESTION

Lipids that are of great biological value for the human body (triacylglycerols, phospholipids, cholesterol, etc.) enter it as food components of biological origin.

For the digestion of lipids in the gastrointestinal tract, the following conditions are necessary:

presence of hydrolyzing lipids lipolytic enzymes;

optimal value for the manifestation of high catalytic activity of lipolytic enzymes pH media (neutral or slightly alkaline);

presence of emulsifiers.

All of the above conditions are created in the human intestine. The salivary glands are not able to produce enzymes that hydrolyze fats, as a result of which no noticeable digestion of fats occurs in the oral cavity. Digestion of fats also does not occur in the stomach of an adult, since pH gastric juice is close to 1.5, and the optimum pH environment for the action of gastric lipolytic enzyme - lipases is in the range of 5.5-7.5. It should be noted that pH gastric juice in newborns is about 5.0, which facilitates the digestion of emulsified triacylglycerols of milk by gastric lipase. In the intestine, hydrochloric acid of gastric juice is neutralized by bicarbonates of intestinal juice and fats are emulsified. Emulsification of lipids is carried out by CO2 bubbles released during the neutralization process with the participation of sodium or potassium salts of bile acids - cholic, 7-deoxycholic, glycincholic, taurocholic and others as surfactants. Bile acids enter the intestines from the gallbladder as part of bile. Emulsification is also facilitated by salts of fatty acids (soaps) formed during the hydrolysis of lipids. But the main role of surfactants in the emulsification of fats belongs to bile acids.

Anions of bile acids sharply reduce the surface tension at the fat-water interface, stabilize the resulting emulsion and form a transport complex with fatty acids, which includes their absorption into the intestinal walls. In addition, bile acids function as activators of lipolytic enzymes.

Triacylglycerols, which make up the bulk of food lipids, are hydrolyzed under the influence of pancreatic lipase, which enters the intestine in an inactive form and is then activated by bile acids. Active lipase has a hydrated hydrophilic region and a hydrophobic head in contact with triacylglycerols at the interface, where step-by-step hydrolysis occurs:

During hydrolysis, in the first stages, ester bonds 1 and 3 are rapidly hydrolyzed, and then the hydrolysis of 2-monoacylglycerol occurs slowly. The resulting 2-monoacylglycerol can then be absorbed by the intestinal wall and used for resynthesis triacylglycerols specific for this type of organism (see below).

They also take part in the hydrolysis of phospholipids phospholipases. Cholesterol esters supplied with food, which are rich in some foods (egg yolk, butter, caviar, etc.), are hydrolyzed cholesterol esterase to free cholesterol and fatty acids. Cholesterol esterase is active only in the presence of bile acids.

The products of hydrolytic breakdown of all dietary lipids are absorbed in the intestine. Glycerol and fatty acids with a short carbon chain (up to 10-12 C atoms) are highly soluble in water and pass into the blood in the form of an aqueous solution. Long-chain fatty acids (more than 14 C atoms) and monoacylglycerols are insoluble in water, therefore they are absorbed with the participation of bile acids, phospholipids and cholesterol, forming a mixture of 12.5: 2.5: 1.0 in the intestine, respectively. As a result, micelles are formed from lipid hydrolysis products surrounded by a hydrophilic shell of cholesterol, phospholipids and bile acids. Subsequently, the micelles disintegrate, and bile acids return to the intestines, completing 5-6 such cycles daily.

Lipids, before entering the lymph, undergo resynthesis, those. conversion to triacylglycerols. The importance of this process lies in the fact that newly synthesized specific fats differ in physical and chemical parameters from dietary lipids and are most suitable for a given organism. Since all differences in the composition of triacylglycerols are determined by the composition of fatty acids, lipid resynthesis uses its own long-chain fatty acids, which are synthesized in the intestine from precursors (only part of the absorbed fatty acids is suitable for resynthesis). Fatty acids form acyl-CoA, and then the acyl residues are transferred to monoacylglycerol with the participation of transacylase, with the sequential formation of di- and triacylglycerols from monoacylglycerol.

Transport of cholesterol and resynthesized lipids is carried out as part of lipoproteins, the protein part of which (apolipoprotein) gives them solubility in aqueous media.

The main metabolic pathways of fatty acids formed during the hydrolysis of dietary triacylglycerols are presented in drawing.

Intracellular lipid hydrolysis

Lipids are continuously renewed in tissues. The half-life of triacylglycerols, which play an important energy role in the body, ranges from 2 to 18 days. Other lipids (phospho-, sphingo-, glycolipids and cholesterol) predominantly act as components of biological membranes and are renewed less intensively. Lipid renewal requires their preliminary intracellular enzymatic hydrolysis - lipolysis.

It is generally accepted that triacylglycerols perform a role in lipid metabolism similar to that played by glycogen in carbohydrate metabolism, and higher fatty acids resemble glucose in their energy value. During physical activity and other conditions of the body that require increased energy expenditure, the consumption of triacylglycerols in adipose tissue as an energy reserve increases. However, only free fatty acids can be used as an energy source. Therefore, triacylglycerols are first hydrolyzed to glycerol and free fatty acids.

acids under the influence of specific tissue lipases. This process is controlled by the central nervous system and is triggered by a number of hormones (adrenaline, norepinephrine, etc.), which activate the hormone-sensitive triacylglycerol lipase. Triacylglycerol lipase breaks down triacylglycerol into diacylglycerol and fatty acid. Then upon action di- And monoacylglycerol lipases further lipolysis occurs to glycerol and fatty acids.

Glycerol formed as a result of lipolysis can participate in gluconeogenesis or be included in glycolysis with the preliminary formation of glycerol-3-phosphate under the influence of glycerol kinase and with the participation of ATF:

Then, under the action of dehydrogenase, glycerol-3-phosphate is converted into triose phosphates, which, in fact, are involved in gluconeogenesis or glycolysis.

Fatty acids, as part of a protein complex with blood albumin, enter the cells of various tissues and organs, where they undergo oxidation.

Biooxidation of fatty acids

The oxidation of fatty acids in organisms is an extremely important process; it can occur at the α-, β- and ω-carbon atoms of fatty acids. The main pathway of fatty acid oxidation in both animal and plant tissues is β-oxidation.

β-Oxidation of fatty acids. β-Oxidation of fatty acids was first studied in 1904 by F. Knoop. It was later found that β-oxidation occurs only in mitochondria. Thanks to the work of F. Linen and his colleagues (1954-1958), the main enzymatic processes of fatty acid oxidation were clarified. In honor of the scientists who discovered this pathway of fatty acid oxidation, the process β-oxidation is called Knoop-Linen cycle.

According to modern concepts, the process of fatty acid oxidation is preceded by their activation in the cytoplasm with the participation acyl-CoA synthetase and using ATP energy:

In the form of acyl-CoA, fatty acids enter mitochondria, in the matrix of which they undergo β-oxidation, which includes the sequence of enzymatic redox reactions listed below.

The first reaction in the fatty acid breakdown pathway is dehydrogenation with the formation of trans-2,3-unsaturated derivatives, catalyzed by various FAD-containing acyl-CoA dehydrogenases:

The second reaction - hydration of the double bond - is catalyzed enoyl-CoA - hydratase:

At the next (third) stage, dehydrogenation of the alcohol fragment occurs, which is carried out by the corresponding dehydrogenase and the oxidized form of the coenzyme NAD:

As a result of oxidation, β-oxo acid is formed, which is why the whole process is called β-oxidation.

The fourth and final reaction, catalyzed thiolase, is accompanied by redox cleavage of the C α -C β bond with the elimination of acetyl-CoA and the addition of a CoA residue at the site of the rupture of the intercarbon bond:

This reaction is called thiolysis and is highly exergonic, so the equilibrium in it is always shifted towards the formation of products.

Consecutive repetition of this cycle of reactions leads to the complete breakdown of fatty acids with an even number of carbon atoms to acetyl-CoA. As a result of this process, acetyl-CoA, FADH 2 and NADH are formed. Next, acetyl-CoA enters the Krebs cycle, and the reduced coenzymes enter the respiratory chain.

The peculiarity of the oxidation of fatty acids with an odd number of carbon atoms is that, along with the usual oxidation products, one molecule CH 3 -CH 2 -CO~SCoA (propionyl-CoA) is formed, which, during the process of carboxylation, is converted into succinyl-CoA, which enters the Krebs cycle.

The characteristics of the oxidation of unsaturated fatty acids are determined by the position and number of double bonds in their molecules. To the point of the double bond, unsaturated fatty acids are oxidized in the same way as saturated fatty acids. If the double bond has the same trans configuration and location as enoyl-CoA, then oxidation proceeds along the usual path. Otherwise, an additional enzyme is involved in the reactions, which moves the double bond to the desired position and changes the configuration of the acid molecule.

β-oxidation of fatty acids releases a large amount of energy. The complete oxidation of one mole of a fatty acid containing 2n carbon atoms produces n moles of acetyl-CoA and (n-1) moles of (FADH 2 + NADH). The oxidation of FADH 2 produces 2ATP, and the oxidation of NADH produces 3ATP. Complete combustion of one mole of acetyl-CoA results in the formation of 12 moles of ATP.

Taking into account the fact that 1 mole of ATP is spent on the activation of a fatty acid, the ATP balance during complete oxidation of a fatty acid with an even number of carbon atoms can be expressed by the following formula:

For example, a mole of palmitic acid containing 16 carbon atoms yields 130 moles of ATP when oxidized. Thus, the energy value of fatty acids is much higher than that of glucose. However, during the oxidation of glucose, oxaloacetate is formed, which facilitates the inclusion of acetyl residues of fatty acids in the Krebs cycle. In this regard, in the biochemical literature there is an expression that “fats burn in the flame of carbohydrates.”

For ease of understanding, the β-oxidation cycle of fatty acids is schematically represented in drawing.

α-Oxidation of fatty acids. Along with β-oxidation, fatty acids with a sufficiently large number of carbon atoms (C13-C18) can undergo α-oxidation. This type of oxidation is especially common in plant tissues, but can also occur in some animal tissues. α-Oxidation is cyclic in nature, and the cycle consists of two reactions.

The first reaction consists of the oxidation of a fatty acid by hydrogen peroxide into the corresponding aldehyde and CO2 with the participation of a specific peroxidase:

As a result of this reaction, the hydrocarbon chain is shortened by one carbon atom.

The essence of the second reaction is the hydration and oxidation of the resulting aldehyde into the corresponding carboxylic acid under the action aldehyde dehydrogenase, containing the oxidized form of the coenzyme NAD:

The α-oxidation cycle then repeats again. Compared to β-oxidation, α-oxidation is energetically less favorable.

The ω-oxo acid is then oxidized to ω-dicarboxylic acid by the action of the corresponding dehydrogenase:

ω-Oxidation of fatty acids. In the liver of animals and in some microorganisms there is an enzyme system that provides ω-oxidation of fatty acids, i.e. oxidation at the terminal CH 3 group, designated by the letter ω. First, under the action of monooxygenase, hydroxylation occurs with the formation of ω-hydroxy acid:

The ω-dicarboxylic acid thus obtained is shortened at either end by β-oxidation reactions.

Digestion of lipids in the intestine.

19.1.1. The main site of lipid digestion is the upper small intestine. The following conditions are necessary for the digestion of lipids:

presence of lipolytic enzymes;

conditions for lipid emulsification;

optimal pH values ​​of the environment (within 5.5 – 7.5).

19.1.2. Various enzymes are involved in the breakdown of lipids. Dietary fats in an adult are broken down mainly by pancreatic lipase; Lipase is also found in intestinal juice and saliva; in infants, lipase is active in the stomach. Lipases belong to the class of hydrolases; they hydrolyze ester bonds -O-CO- to form free fatty acids, diacylglycerols, monoacylglycerols, glycerol (Figure 19.1).

Figure 19.1. Scheme of fat hydrolysis.

Glycerophospholipids supplied with food are exposed to specific hydrolases - phospholipases, which cleave ester bonds between the components of phospholipids. The specificity of the action of phospholipases is shown in Figure 19.2.

Figure 19.2. Specificity of the action of enzymes that break down phospholipids.

The products of phospholipid hydrolysis are fatty acids, glycerol, inorganic phosphate, nitrogenous bases (choline, ethanolamine, serine).

Dietary cholesterol esters are hydrolyzed by pancreatic cholesterol esterase to form cholesterol and fatty acids.

19.1.3. Understand the structural features of bile acids and their role in the digestion of fats. Bile acids are the end product of cholesterol metabolism and are formed in the liver. These include: cholic (3,7,12-trioxycholanic), chenodeoxycholic (3,7-dioxycholanic) and deoxycholic (3, 12-dioxycholanic) acids (Figure 19.3, a). The first two are primary bile acids (formed directly in hepatocytes), deoxycholic acid is secondary (as it is formed from primary bile acids under the influence of intestinal microflora).

In bile, these acids are present in conjugated form, i.e. in the form of compounds with glycine H 2 N -CH 2 -COOH or taurine H 2 N -CH 2 -CH 2 - SO 3 H (Figure 19.3, b).

Figure 19.3. The structure of unconjugated (a) and conjugated (b) bile acids.

19.1.4. Bile acids have amphiphilic properties: the hydroxyl groups and side chain are hydrophilic, the cyclic structure is hydrophobic. These properties determine the participation of bile acids in the digestion of lipids:

1) bile acids are capable of emulsifying fats; their molecules, with their non-polar part, are adsorbed on the surface of fat droplets, while at the same time hydrophilic groups interact with the surrounding aqueous environment. As a result, the surface tension at the interface between the lipid and aqueous phases decreases, as a result of which large fat droplets are broken into smaller ones;

2) bile acids, along with bile colipase, participate in the activation of pancreatic lipase, shifting its pH optimum to the acidic side;

3) bile acids form water-soluble complexes with hydrophobic products of fat digestion, which facilitates their absorption into the wall of the small intestine.

Bile acids, which penetrate into the enterocytes during absorption along with hydrolysis products, enter the liver through the portal system. These acids can be re-secreted with bile into the intestines and participate in the processes of digestion and absorption. Such enterohepatic circulation of bile acids can occur up to 10 or more times a day.

19.1.5. Features of absorption of fat hydrolysis products in the intestine are presented in Figure 19.4. During the digestion of food triacylglycerols, about 1/3 of them are completely broken down to glycerol and free fatty acids, approximately 2/3 are partially hydrolyzed to form mono- and diacylglycerols, and a small part is not broken down at all. Glycerol and free fatty acids with a chain length of up to 12 carbon atoms are soluble in water and penetrate into the enterocytes, and from there through the portal vein into the liver. Longer fatty acids and monoacylglycerols are absorbed with the participation of conjugated bile acids, which form micelles. Undigested fats can apparently be absorbed by the cells of the intestinal mucosa by pinocytosis. Water-insoluble cholesterol, like fatty acids, is absorbed in the intestine in the presence of bile acids.

Figure 19.4. Digestion and absorption of acylglycerols and fatty acids.

Section 19.2

Resynthesis of lipids in the intestinal wall and the formation of chylomicrons.

19.2.1. In the cells of the intestinal mucosa, body-specific lipids are synthesized from the products of digestion of dietary lipids (the fatty acid composition of such lipids corresponds to the fatty acid composition of endogenous fats). The resynthesis process produces mainly triacylglycerols, as well as phospholipids and cholesterol esters.

19.2.2. Transport of resynthesized lipids from the intestinal wall occurs in the form of chylomicrons. Chylomicrons are complex particles consisting of lipids and proteins. They have a spherical shape, their diameter is about 1 micron. The lipid core of chylomicrons is formed by triacylglycerols (80% or more) and cholesterol esters. The chylomicron shell is composed of amphiphilic compounds - proteins (apolipoproteins), phospholipids and free cholesterol (see Figure 19.5).

Figure 19.5. Diagram of the structure of a chylomicron.

Chylomicrons are a transport form of lipids from the intestine to other organs and tissues; they enter from the mucosal cells first into the lymph and then into the blood. Endothelial cells of the blood capillaries of adipose tissue, liver cells and other organs contain the enzyme lipoprotein lipase. Lipoprotein lipase acts on chylomicrons, hydrolyzing their constituent fats (see further 19.5.2 and Figure 19.9).

19.2.3. Free fatty acids (FFA) formed during the catabolism of chylomicrons are transported in the blood in combination with albumin proteins. Blood FFAs are absorbed and used by the cells of adipose tissue and other organs.

FFAs also enter the blood as a result of lipolysis of triacylglycerols in adipose tissue. These lipolysis reactions are catalyzed by tissue lipase. The activity of this enzyme is regulated by hormones. For example, the hormones adrenaline and glucagon activate lipase and enhance lipolysis processes, while the hormone insulin helps slow down lipolysis in adipose tissue.

The main pathways for the formation and use of free fatty acids are presented in Figure 19.6.

Figure 19.6. The main pathways of formation and use of fatty acids.

The first two stages of lipid digestion, emulsification And hydrolysis, occur almost simultaneously. At the same time, hydrolysis products are not removed, but remaining in the lipid droplets, they facilitate further emulsification and the work of enzymes.

Digestion in the mouth

In adults, lipid digestion does not occur in the oral cavity, although prolonged chewing of food contributes to partial emulsification of fats.

Digestion in the stomach

In an adult, the stomach's own lipase does not play a significant role in the digestion of lipids due to its small amount and the fact that its optimum pH is 4.5-5.5. The lack of emulsified fats in regular foods (except milk) also affects this.

However, in adults, a warm environment and gastric peristalsis causes some emulsification fat At the same time, even low active lipase breaks down small amounts of fat, which is important for the further digestion of fats in the intestines, because the presence of at least a minimal amount of free fatty acids facilitates the emulsification of fats in the duodenum and stimulates the secretion of pancreatic lipase.

Digestion in the intestines

Under the influence peristalsis Gastrointestinal tract and constituent components bile edible fat is emulsified. Formed during digestion lysophospholipids They are also a good surfactant, so they promote further emulsification of dietary fats and the formation of micelles. The droplet size of such a fat emulsion does not exceed 0.5 microns.

Hydrolysis of CS esters is carried out cholesterol esterase pancreatic juice.

Digestion of TAG in the intestine is carried out under the influence of pancreatic lipase with an optimum pH of 8.0-9.0. It enters the intestines in the form prolipases, for the manifestation of its activity, colipase is required, which helps lipase to locate on the surface of the lipid droplet.

Colipase, in turn, is activated by trypsin and then forms a complex with lipase in a 1:1 ratio. Pancreatic lipase removes fatty acids bound to the C1 and C3 carbon atoms of glycerol. As a result of its work, 2-monoacylglycerols (2-MAG) remain, which are absorbed or converted monoglycerol isomerase in 1-MAG. The latter is hydrolyzed to glycerol and fatty acid. Approximately 3/4 of TAG after hydrolysis remains in the form of 2-MAG and only 1/4 of TAG is completely hydrolyzed.

Complete enzymatic hydrolysis of triacylglycerol

IN pancreatic juice also contains trypsin-activated phospholipase A 2, which cleaves fatty acid from C 2 in phospholipids; the activity of phospholipase C and lysophospholipases.

The action of phospholipase A 2 and lysophospholipase using the example of phosphatidylcholine

IN intestinal juice also has phospholipase A 2 and phospholipase C activity.

For all of these hydrolytic enzymes to work in the intestine, Ca 2+ ions are required to facilitate the removal of fatty acids from the catalytic zone.

Points of action of phospholipases

Micelle formation

As a result of the action of pancreatic and intestinal juice enzymes on emulsified fats, 2-monoacylglycerol s, free fatty acids and free cholesterol, forming micellar-type structures (size already about 5 nm). Free glycerol is absorbed directly into the blood.

The role of lipids in nutrition

Lipids are an essential part of a balanced human diet. It is generally accepted that with a balanced diet, the ratio of proteins, lipids and carbohydrates in the diet is approximately 1: 1: 4. On average, about 80 g of fats of animal and plant origin enter the body of an adult with food every day. In old age, as well as with little physical activity, the need for fat decreases; in cold climates and with heavy physical work, it increases.

The value of fats as a food product is very diverse. First of all, fats in human nutrition have an important energy value. The high calorie content of fats compared to proteins and carbohydrates gives them special nutritional value when the body expends large amounts of energy. It is known that 1 g of fats, when oxidized in the body, gives 38.9 kJ (9.3 kcal), while 1 g of protein or carbohydrates - 17.2 kJ (4.1 kcal). It should also be remembered that fats are solvents for vitamins A, D, E, etc., and therefore the body’s supply of these vitamins largely depends on the intake of fats in food. In addition, some polyunsaturated acids (linoleic, linolenic, arachidonic) are introduced into the body with fats, which are classified as essential fatty acids, because human tissues and a number of animals have lost the ability to synthesize them. These acids are conventionally combined into a group called “vitamin F”.

Finally, with fats the body receives a complex of biologically active substances, such as phospholipids, sterols, etc., which play an important role in metabolism.

Digestion and absorption of lipids

Breakdown of fats in the gastrointestinal tract. Saliva does not contain fat-breaking enzymes. Consequently, fats do not undergo any changes in the oral cavity. In adults, fats also pass through the stomach without any special changes, since the lipase contained in small quantities in the gastric juice of adults and mammals is inactive. The pH value of gastric juice is about 1.5, and the optimal pH value for gastric lipase is in the range of 5.5-7.5. In addition, lipase can actively hydrolyze only pre-emulsified fats; in the stomach, there are no conditions for emulsifying fats.

The digestion of fats in the stomach cavity plays an important role in the digestion process in children, especially infants. It is known that the pH of gastric juice in infants is about 5.0, which facilitates the digestion of emulsified milk fat by gastric lipase. In addition, there is reason to believe that with long-term consumption of milk as the main food product in infants, an adaptive increase in the synthesis of gastric lipase is observed.

Although no significant digestion of food fats occurs in the stomach of an adult, partial destruction of the lipoprotein complexes of food cell membranes is still observed in the stomach, which makes fats more accessible for the subsequent action of pancreatic juice lipase on them. In addition, a slight breakdown of fats in the stomach leads to the appearance of free fatty acids, which, when entering the intestines, contribute to the emulsification of fats there.

The breakdown of fats that make up food occurs in humans and mammals mainly in the upper parts of the small intestine, where there are very favorable conditions for the emulsification of fats.

After the chyme enters the duodenum, here, first of all, the hydrochloric acid of the gastric juice that enters the intestine with food, bicarbonates contained in the pancreatic and intestinal juices is neutralized. The bubbles of carbon dioxide released during the decomposition of bicarbonates contribute to good mixing of the food gruel with digestive juices. At the same time, fat emulsification begins. The most powerful emulsifying effect on fats, undoubtedly, is exerted by bile salts, which enter the duodenum with bile in the form of sodium salts, most of which are conjugated with glycine or taurine. Bile acids are the main end product of cholesterol metabolism.

The main stages of the formation of bile acids, in particular cholic acid, from cholesterol can be represented as follows. The process begins with the hydroxylation of cholesterol at the 7th α-position, i.e., with the inclusion of a hydroxyl group at position 7 and the formation of 7-hydroxycholesterol. Then, through a series of steps, 3,7,12-trihydroxycoprostanoic acid is formed, the side chain of which undergoes β-oxidation. In the final stage, propionic acid (in the form of propionyl-CoA) is separated and the side chain is shortened. A large number of liver enzymes and coenzymes take part in all these reactions.

By their chemical nature, bile acids are derivatives of cholanic acid. Human bile mainly contains cholic (3,7,12-trioxycholanic), deoxycholic (3,12-dihydroxycholanic) and chenodeoxycholic (3,7-dihydroxycholanic) acids.

In addition, human bile contains lithocholic (3-hydroxycholanic) acid in small (trace) quantities, as well as allocholic and ureodeoxycholic acids - stereoisomers of cholic and chenodeoxycholic acids.

As already noted, bile acids are present in bile in conjugated form, i.e. in the form of glycocholic, glycodeoxycholic, glycochenodeoxycholic (about 2/3-4/3 of all bile acids) or taurocholic, taurodeoxycholic and taurochenodeoxycholic (about 1/5-1 /3 of all bile acids). These compounds are sometimes called paired compounds, since they consist of two components - bile acid and glycine, or bile acid and taurine.

Note that the ratios between the conjugates of these two types may vary depending on the nature of the food: if carbohydrates predominate in it, the relative content of glycine conjugates increases, and with a high-protein diet, the content of taurine conjugates increases. The structure of these conjugates can be presented as follows:

It is believed that only the combination: bile salt + unsaturated fatty acid + monoglyceride can provide the required degree of fat emulsification. Bile salts dramatically reduce the surface tension at the fat/water interface, due to which they not only facilitate emulsification, but also stabilize the already formed emulsion.

Bile acids also play an important role as a kind of activator of pancreatic lipase 1, under the influence of which fat is broken down in the intestines. Lipase produced in the pancreas breaks down triglycerides that are in an emulsified state. It is believed that the activating effect of bile acids on lipase is expressed in a shift in the optimum action of this enzyme from pH 8.0 to 6.0, i.e., to the pH value that is more constantly maintained in the duodenum during the digestion of fatty foods. The specific mechanism of lipase activation by bile acids is still unclear.

1 However, there is an opinion that lipase activation does not occur under the influence of bile acids. Pancreatic juice contains a lipase precursor, which is activated in the intestinal lumen by forming a complex with colipase (cofactor) in a molar ratio of 2: 1. This helps to shift the pH optimum from 9.0 to 6.0 and prevent denaturation of the enzyme. It has also been established that the rate of hydrolysis catalyzed by lipase is not significantly affected by either the degree of unsaturation of fatty acids or the length of the hydrocarbon chain (from C 12 to C 18). Calcium ions accelerate hydrolysis mainly because they form insoluble soaps with the liberated fatty acids, i.e., they practically shift the reaction in the direction of hydrolysis.

There is reason to believe that there are two types of pancreatic lipase: one of them is specific for the ester bonds in positions 1 and 3 of the triglyceride, and the other hydrolyzes the bonds in position 2. Complete hydrolysis of triglycerides occurs in stages: first, bonds 1 and 3 are quickly hydrolyzed, and then hydrolysis of the 2-monoglyceride occurs slowly (scheme).

It should be noted that intestinal lipase is also involved in the breakdown of fats, but its activity is low. In addition, this lipase catalyzes the hydrolytic breakdown of monoglycerides and does not act on di- and triglycerides. Thus, practically the main products formed in the intestines during the breakdown of dietary fats are fatty acids, monoglycerides and glycerol.

Absorption of fats in the intestine. Absorption occurs in the proximal small intestine. Thinly emulsified fats (the size of fat droplets of the emulsion should not exceed 0.5 microns) can be partially absorbed through the intestinal wall without prior hydrolysis. However, the bulk of the fat is absorbed only after it is broken down by pancreatic lipase into fatty acids, monoglycerides and glycerol. Fatty acids with a short carbon chain (less than 10 C atoms) and glycerol, being highly soluble in water, are freely absorbed in the intestine and enter the blood of the portal vein, from there to the liver, bypassing any transformations in the intestinal wall. The situation is more complicated with long-carbon chain fatty acids and monoglycerides. The absorption of these compounds occurs with the participation of bile and mainly the bile acids included in its composition. Bile contains bile salts, phospholipids and cholesterol in a ratio of 12.5:2.5:1.0. Long-chain fatty acids and monoglycerides in the intestinal lumen form micelles (micellar solution) that are stable in an aqueous environment with these compounds. The structure of these micelles is such that their hydrophobic core (fatty acids, glycerides, etc.) is surrounded on the outside by a hydrophilic shell of bile acids and phospholipids. Micelles are approximately 100 times smaller than the smallest emulsified fat droplets. As part of micelles, higher fatty acids and monoglycerides are transferred from the site of fat hydrolysis to the absorption surface of the intestinal epithelium. There is no consensus regarding the mechanism of absorption of fat micelles. Some researchers believe that as a result of so-called micellar diffusion, and possibly pinocytosis, micelles penetrate into the epithelial cells of the villi as a whole particle. Here the breakdown of fat micelles occurs; in this case, bile acids immediately enter the bloodstream and enter the liver through the portal vein system, from where they are again secreted as part of bile. Other researchers admit the possibility that only the lipid component of fat micelles passes into the villi cells. And bile salts, having fulfilled their physiological role, remain in the intestinal lumen. And only then, in the overwhelming majority, they are absorbed into the blood (in the ileum), enter the liver and are then excreted with bile. Thus, both researchers recognize that there is a constant circulation of bile acids between the liver and intestines. This process is called hepatic-intestinal (enterohepatic) circulation.

Using the labeled atom method, it was shown that bile contains only a small part of bile acids (10-15% of the total) newly synthesized by the liver, i.e. the bulk of bile acids in bile (85-90%) are bile acids , reabsorbed in the intestine and re-secreted as part of bile. It has been established that in humans the total pool of bile acids is approximately 2.8-3.5 g; at the same time, they make 5-6 revolutions per day.

Resynthesis of fats in the intestinal wall. The intestinal wall synthesizes fats that are largely specific to a given animal species and differ in nature from dietary fat. To a certain extent, this is ensured by the fact that in the synthesis of triglycerides (as well as phospholipids) in the intestinal wall, along with exogenous and endogenous fatty acids, they take part. However, the ability to carry out the synthesis of fat specific to a given animal species in the intestinal machine is still limited. A. N. Lebedev showed that when feeding an animal, especially a previously starved one, large amounts of foreign fat (for example, flaxseed oil or camel fat), part of it is found in the fatty tissues of the animal unchanged. Fat depots are most likely the only tissue where foreign fats can be deposited. Lipids that make up the protoplasm of cells of other organs and tissues are highly specific; their composition and properties depend little on dietary fats.

The mechanism of resynthesis of triglycerides in the cells of the intestinal wall is generally reduced to the following: initially, their active form, acyl-CoA, is formed from fatty acids, after which acylation of monoglycerides occurs with the formation of first diglycerides and then triglycerides:

Thus, in the cells of the intestinal epithelium of higher animals, monoglycerides formed in the intestine during the digestion of food can be acylated directly, without intermediate stages.

However, the epithelial cells of the small intestine contain enzymes - monoglyceride lipase, which breaks down monoglyceride into glycerol and fatty acid, and glycerol kinase, which can convert glycerol (formed from monoglyceride or absorbed from the intestine) into glycerol-3-phosphate. The latter, interacting with the active form of fatty acid - acyl-CoA, produces phosphatidic acid, which is then used for the resynthesis of triglycerides and especially glycerophospholipids (see details below).

Digestion and absorption of glycerophospholipids and cholesterol. Glycerophospholipids introduced with food are exposed in the intestine to specific hydrolytic enzymes that break the ether bonds between the components that make up the phospholipids. It is generally accepted that in the digestive tract, the breakdown of glycerophospholipids occurs with the participation of phospholipases secreted with pancreatic juice. Below is a diagram of the hydrolytic cleavage of phosphatidylcholine:

There are several types of phospholipases.

• Phospholipase A 1 hydrolyzes the ester bond at position 1 of the glycerophospholipid, as a result of which one molecule of fatty acid is cleaved and, for example, when phosphatidylcholine is broken down, 2-acylglycerylphosphorylcholine is formed.
• Phospholipase A 2 , formerly simply called phospholipase A, catalyzes the hydrolytic cleavage of the fatty acid at position 2 of glycerophospholipid. The resulting products are called lysophosphatidylcholine and lysophosphatidylethanolamine. They are toxic and cause destruction of cell membranes. The high activity of phospholipase A 2 in the venom of snakes (cobra, etc.) and scorpions leads to the fact that when they bite, red blood cells are hemolyzed.

  Phospholipase A 2 of the pancreas enters the cavity of the small intestine in an inactive form and only after exposure to trypsin, leading to the cleavage of the heptapeptide from it, becomes active. The accumulation of lysophospholipids in the intestine can be eliminated if both phospholipases act simultaneously on glycerophospholipids: A 1 and A 2. As a result, a product that is non-toxic to the body is formed (for example, when phosphatidylcholine is broken down - glycerylphosphorylcholine).

• Phospholipase C causes hydrolysis of the bond between phosphoric acid and glycerol, and phospholipase D cleaves the ester bond between the nitrogenous base and phosphoric acid to form the free base and phosphatidic acid.

So, as a result of the action of phospholipases, glycerophospholipids are broken down to form glycerol, higher fatty acids, nitrogenous base and phosphoric acid.

It should be noted that a similar mechanism for the breakdown of glycerophospholipids also exists in body tissues; This process is catalyzed by tissue phospholipases. Note that the sequence of reactions for the cleavage of glycerophospholipids into individual components is still unknown.

We have already discussed the mechanism of absorption of higher fatty acids and glycerol. Phosphoric acid is absorbed by the intestinal wall mainly in the form of sodium or potassium salts. Nitrogenous bases (choline and ethanolamine) are absorbed in the form of their active forms.

As already noted, resynthesis of glycerophospholipids occurs in the intestinal wall. Necessary components for synthesis: higher fatty acids, glycerol, phosphoric acid, organic nitrogenous bases (choline or ethanolamine) enter the epithelial cell upon absorption from the intestinal cavity, since they are formed during the hydrolysis of dietary fats and lipids; These components are partially delivered to the intestinal epithelial cells through the bloodstream from other tissues. Resynthesis of glycerophospholipids proceeds through the stage of formation of phosphatidic acid.

As for cholesterol, it enters the human digestive organs mainly with egg yolk, meat, liver, and brain. The adult body daily receives 0.1-0.3 g of cholesterol contained in food products either in the form of free cholesterol or in the form of its esters (cholesterides). Cholesterol esters are broken down into cholesterol and fatty acids with the participation of a special enzyme in pancreatic and intestinal juices - cholesterol esterase. Water-insoluble cholesterol, like fatty acids, is absorbed in the intestine only in the presence of bile acids.

Chylomicron formation and lipid transport. Triglycerides and phospholipids resynthesized in intestinal epithelial cells, as well as cholesterol entering these cells from the intestinal cavity (here it can be partially esterified) combine with a small amount of protein and form relatively stable complex particles - chylomicrons (CM). The latter contain about 2% protein, 7% phospholipids, 8% cholesterol and its esters and over 80% triglycerides. The diameter of the CM ranges from 100 to 5000 nm. Due to the large particle size, CMs are not able to penetrate from the intestinal endothelial cells into the blood capillaries and diffuse into the intestinal lymphatic system, and from it into the thoracic lymphatic duct. Then, from the thoracic lymphatic duct, HMs enter the bloodstream, i.e., with their help, exogenous triglycerides, cholesterol and partially phospholipids are transported from the intestine through the lymphatic system into the blood. Already 1-2 hours after ingestion of food containing lipids, nutritional hyperlipemia is observed. This is a physiological phenomenon, characterized primarily by an increase in the concentration of triglycerides in the blood and the appearance of CM in it. The peak of nutritional hyperlipemia occurs 4-6 hours after ingestion of fatty foods. Usually, 10-12 hours after eating, the triglyceride content returns to normal values, and CM completely disappear from the bloodstream.

It is known that the liver and adipose tissue play the most significant role in the further fate of CM. The latter diffuse freely from the blood plasma into the intercellular spaces of the liver (sinusoids). It is assumed that the hydrolysis of CM triglycerides occurs both inside liver cells and on their surface. As for adipose tissue, chylomicrons are not able (due to their size) to penetrate its cells. In this regard, CM triglycerides undergo hydrolysis on the surface of the capillary endothelium of adipose tissue with the participation of the enzyme lipoprotein lipase, which is closely associated with the surface of the capillary endothelium. As a result, fatty acids and glycerol are formed. Some of the fatty acids pass into the fat cells, and some bind to serum albumin and are carried away with its current. Adipose tissue and glycerol can leave the bloodstream.

The breakdown of CM triglycerides in the liver and in the blood capillaries of adipose tissue actually leads to the cessation of the existence of CM.

Intermediate lipid metabolism. Includes the following main processes: the breakdown of triglycerides in tissues with the formation of higher fatty acids and glycerol, the mobilization of fatty acids from fat depots and their oxidation, the formation of acetone bodies (ketone bodies), the biosynthesis of higher fatty acids, triglycerides, glycerophospholipids, sphingolipids, cholesterol, etc. d.

Intracellular lipolysis

The main endogenous source of fatty acids used as “fuel” is reserve fat contained in adipose tissue. It is generally accepted that triglycerides in fat depots play the same role in lipid metabolism as liver glycogen in carbohydrate metabolism, and higher fatty acids in their role resemble glucose, which is formed during the phosphorolysis of glycogen. During physical work and other conditions of the body that require increased energy expenditure, the consumption of adipose tissue triglycerides as an energy reserve increases.

Since only free, i.e. non-esterified, fatty acids can be used as energy sources, triglycerides are first hydrolyzed using specific tissue enzymes - lipases - to glycerol and free fatty acids. The last of the fat depots can pass into the blood plasma (mobilization of higher fatty acids), after which they are used by the tissues and organs of the body as energy material.

Adipose tissue contains several lipases, of which the most important are triglyceride lipase (the so-called hormone-sensitive lipase), diglyceride lipase and monoglyceride lipase. The activity of the last two enzymes is 10-100 times higher than the activity of the first. Triglyceride lipase is activated by a number of hormones (for example, adrenaline, norepinephrine, glucagon, etc.), while diglyceride lipase and monoglyceride lipase are insensitive to their action. Triglyceride lipase is a regulatory enzyme.

It has been established that hormone-sensitive lipase (triglyceride lipase) is found in adipose tissue in an inactive form and is activated by cAMP. As a result of the influence of hormones, the primary cellular receptor modifies its structure, and in this form it is able to activate the enzyme adenylate cyclase, which in turn stimulates the formation of cAMP from ATP. The resulting cAMP activates the enzyme protein kinase, which, by phosphorylating inactive triglyceride lipase, converts it into an active form (Fig. 96). Active triglyceride lipase breaks down triglyceride (TG) into diglyceride (DG) and fatty acid (FA). Then, under the action of di- and monoglyceride lipases, the final products of lipolysis are formed - glycerol (GL) and free fatty acids, which enter the bloodstream.

Free fatty acids bound to plasma albumin in the form of a complex enter organs and tissues through the bloodstream, where the complex disintegrates, and the fatty acids undergo either β-oxidation, or part of them is used for the synthesis of triglycerides (which are then used for the formation of lipoproteins), glycerophospholipids, sphingolipids and other compounds, as well as the esterification of cholesterol.

Another source of fatty acids is membrane phospholipids. In the cells of higher animals, metabolic renewal of phospholipids continuously occurs, during which free fatty acids are formed (a product of the action of tissue phospholipases).

Digestion of proteins

Proteolytic enzymes involved in the digestion of proteins and peptides are synthesized and secreted into the cavity of the digestive tract in the form of proenzymes, or zymogens. Zymogens are inactive and cannot digest the cells' own proteins. Proteolytic enzymes are activated in the intestinal lumen, where they act on food proteins.

In human gastric juice there are two proteolytic enzymes - pepsin and gastrixin, which are very similar in structure, which indicates their formation from a common precursor.

Pepsin is formed in the form of a proenzyme - pepsinogen - in the main cells of the gastric mucosa. Several pepsinogens with similar structures have been isolated, from which several varieties of pepsin are formed: pepsin I, II (IIa, IIb), III. Pepsinogens are activated with the help of hydrochloric acid, secreted by the parietal cells of the stomach, and autocatalytically, that is, with the help of the resulting pepsin molecules.

Pepsinogen has a molecular weight of 40,000. Its polypeptide chain includes pepsin (molecular weight 34,000); a fragment of a polypeptide chain that is a pepsin inhibitor (molecular weight 3100), and a residual (structural) polypeptide. The pepsin inhibitor has sharply basic properties, as it consists of 8 lysine residues and 4 arginine residues. Activation consists of the cleavage of 42 amino acid residues from the N-terminus of pepsinogen; First, the residual polypeptide is cleaved off, followed by the pepsin inhibitor.

Pepsin belongs to carboxyproteinases containing dicarboxylic amino acid residues in the active center with an optimum pH of 1.5-2.5.

Pepsin substrates are proteins, either native or denatured. The latter are easier to hydrolyze. Denaturation of food proteins is ensured by cooking or the action of hydrochloric acid. The following should be noted biological functions of hydrochloric acid:

1. pepsinogen activation;
2. creating an optimum pH for the action of pepsin and gastricsin in gastric juice;
3. denaturation of food proteins;
4. antimicrobial action.

The own proteins of the stomach walls are protected from the denaturing effect of hydrochloric acid and the digestive action of pepsin by a mucous secretion containing glycoproteins.

Pepsin, being an endopeptidase, quickly cleaves internal peptide bonds in proteins formed by the carboxyl groups of aromatic amino acids - phenylalanine, tyrosine and tryptophan. The enzyme hydrolyzes peptide bonds between leucine and dicarboxylic amino acids more slowly: in the polypeptide chain.

Gastricin close to pepsin in molecular weight (31,500). Its optimum pH is about 3.5. Gastricsin hydrolyzes peptide bonds formed by dicarboxylic amino acids. The pepsin/gastricsin ratio in gastric juice is 4:1. In case of peptic ulcer, the ratio changes in favor of gastricsin.

The presence of two proteinases in the stomach, of which pepsin acts in a strongly acidic environment, and gastrixin in a moderately acidic environment, allows the body to more easily adapt to dietary patterns. For example, vegetable and dairy nutrition partially neutralizes the acidic environment of gastric juice, and the pH favors the digestive action of gastricsin rather than pepsin. The latter breaks down the bonds in food protein.

Pepsin and gastrixin hydrolyze proteins into a mixture of polypeptides (also called albumoses and peptones). The depth of protein digestion in the stomach depends on the length of time food is in it. Usually this is a short period, so the bulk of the proteins are broken down in the intestines.

Intestinal proteolytic enzymes. Proteolytic enzymes enter the intestine from the pancreas in the form of proenzymes: trypsinogen, chymotrypsinogen, procarboxypeptidases A and B, proelastase. Activation of these enzymes occurs through partial proteolysis of their polypeptide chain, i.e., the fragment that masks the active center of proteinases. The key process of activation of all proenzymes is the formation of trypsin (Fig. 1).

Trypsinogen coming from the pancreas is activated by enterokinase, or enteropeptidase, which is produced by the intestinal mucosa. Enteropeptidase is also secreted as a kinase gene precursor, which is activated by bile protease. Activated enteropeptidase quickly converts trypsinogen into trypsin; trypsin carries out slow autocatalysis and quickly activates all other inactive precursors of pancreatic juice proteases.

The mechanism of trypsinogen activation is the hydrolysis of one peptide bond, resulting in the release of an N-terminal hexapeptide called trypsin inhibitor. Next, trypsin, breaking peptide bonds in other proenzymes, causes the formation of active enzymes. In this case, three types of chymotrypsin, carboxypeptidase A and B, and elastase are formed.

Intestinal proteinases hydrolyze peptide bonds of food proteins and polypeptides formed after the action of gastric enzymes to free amino acids. Trypsin, chymotrypsins, and elastase, being endopeptidases, promote the rupture of internal peptide bonds, breaking up proteins and polypeptides into smaller fragments.

• Trypsin hydrolyzes peptide bonds formed mainly by the carboxyl groups of lysine and arginine; it is less active against peptide bonds formed by isoleucine.
• Chymotrypsins are most active against peptide bonds, in the formation of which tyrosine, phenylalanine, and tryptophan take part. In terms of specificity of action, chymotrypsin is similar to pepsin.
• Elastase hydrolyzes those peptide bonds in polypeptides where proline is located.
• Carboxypeptidase A is a zinc-containing enzyme. It cleaves C-terminal aromatic and aliphatic amino acids from polypeptides, while carboxypeptidase B cleaves only C-terminal lysine and arginine residues.

Enzymes that hydrolyze peptides are also present in the intestinal mucosa, and although they can be secreted into the lumen, they function predominantly intracellularly. Therefore, hydrolysis of small peptides occurs after they enter the cells. Among these enzymes are leucine aminopeptidase, which is activated by zinc or manganese, as well as cysteine, and releases N-terminal amino acids, as well as dipeptidases, which hydrolyze dipeptides into two amino acids. Dipeptidases are activated by cobalt, manganese and cysteine ​​ions.

A variety of proteolytic enzymes leads to the complete breakdown of proteins into free amino acids, even if the proteins were not previously exposed to pepsin in the stomach. Therefore, patients after surgery for partial or complete removal of the stomach retain the ability to absorb food proteins.

Mechanism of digestion of complex proteins

The protein part of complex proteins is digested in the same way as simple proteins. Their prosthetic groups are hydrolyzed depending on their structure. The carbohydrate and lipid components, after they are cleaved from the protein part, are hydrolyzed by amylolytic and lipolytic enzymes. The porphyrin group of chromoproteins is not cleaved.

Of interest is the process of breakdown of nucleoproteins, which are rich in some foods. The nucleic component is separated from the protein in the acidic environment of the stomach. In the intestine, polynucleotides are hydrolyzed by intestinal and pancreatic nucleases.

RNA and DNA are hydrolyzed under the action of pancreatic enzymes - ribonuclease (RNase) and deoxyribonuclease (DNase). Pancreatic RNase has an optimum pH of about 7.5. It cleaves internal internucleotide bonds in RNA. In this case, shorter polynucleotide fragments and cyclic 2,3-nucleotides are formed. Cyclic phosphodiester bonds are hydrolyzed by the same RNase or intestinal phosphodiesterase. Pancreatic DNase hydrolyzes internucleotide bonds in DNA supplied with food.

The products of hydrolysis of polynucleotides - mononucleotides are exposed to the action of enzymes of the intestinal wall: nucleotidase and nucleosidase:

These enzymes have relative group specificity and hydrolyze both ribonucleotides and ribonucleosides and deoxyribonucleotides and deoxyribonucleosides. Nucleosides, nitrogenous bases, ribose or deoxyribose, H 3 PO 4 are absorbed.